Lighting device having a remote wavelength converting layer

ABSTRACT

According to an aspect of the present invention, a lighting device ( 2 ) is provided. The lighting device ( 2 ) comprises a wavelength converting layer ( 21 ) having a curved shape and a light source ( 22 ) arranged to emit light towards the wavelength converting layer ( 21 ). The wavelength converting layer ( 21 ) intersects a plane extending through the light source ( 22 ) and being parallel with the optical axis of the light source ( 22 ), at a curve given, in a polar coordinate system centered at the light source ( 22 ), by the equation: R(φ)=k·I(φ) 1/2 ±D, wherein k is a constant, 0 is an angle with respect to said optical axis, /(φ) is a function defining a luminous intensity profile of the light source and D is a deviation ranging from zero to 20% of the maximum value of said curve, R max . The present invention is advantageous in that the lighting device ( 2 ) has a more uniform color distribution of emitted light across the wavelength converting layer ( 21 ) and the risk of color gradients and artifacts is reduced.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C.§371 of International Application No. PCT/IB13/054388, filed on May 28,2013, which claims the benefit of U.S. Provisional Patent ApplicationNo. 61/655,538, filed on Jun. 5, 2012. These applications are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The present invention generally relates to the field of lighting deviceshaving remote wavelength converting layers.

BACKGROUND OF THE INVENTION

Wavelength converting materials, such as phosphors, are used for tuningthe color of light emitting diode (LED) based light sources. Phosphorsin combination with blue LEDs are used to produce white light. Dependingon the type of phosphors and the amount of conversion, the color can betuned to achieve a desired color such as cool white or warm white. Thewhite light is produced by a combination of transmitted (unconverted)blue light and converted, often yellowish, light.

When the phosphor is arranged in a substrate or layer separate, i.e. ata certain distance, from the LED, it is referred to as a remote phosphorlayer. Such a remote phosphor layer may be provided directly in an outerenvelope of the lighting device or as a separate layer inside theenvelope. Examples of such lighting devices are shown in CN201606695 andEP2293355.

A problem with remote phosphor layers is that the color distribution oflight emitted from the exit surface, i.e. the surface of the remotephosphor layer from which light is emitted, may be non-uniform. This isin particular the case in LED-based tube lamps having e.g. blue LEDs anda phosphor mixture in the curved envelope, wherein yellow lines arevisible at the edges of the envelope at angles close to ±90° withrespect to an optical axis of the lamp.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome this problem and toprovide a lighting device with a more uniform color distribution ofemitted light across the wavelength converting layer.

According to an aspect of the present invention, this and other objectsare achieved by a lighting device as defined in the independent claim.Embodiments of the invention are defined in the dependent claims.

According to an aspect of the present invention, a lighting device isprovided. The lighting device comprises a wavelength converting layerhaving a curved shape and a light source arranged to emit light towardsthe wavelength converting layer. The wavelength converting layerintersects a plane extending through the light source and being parallelwith the optical axis of the light source, at a curve given, in a polarcoordinate system centered at the light source, by the equation:R(φ)=k·I(φ)^(1/2) ±D  (Equation 1)wherein k is a constant, φ is an angle with respect to the optical axis,I(φ) is a function defining a luminous intensity profile of the lightsource and D is a deviation ranging from zero to 20% of the maximumvalue of said curve, R_(max).

Another way of defining the wavelength converting layer is that theoutline of the shape of the wavelength converting layer is defined by acurve whose radius R is, in a polar coordinate system centered at thelight source, expressed by Equation 1, wherein k is a constant, φ is anangle with respect to the optical axis, I(φ) is a function defining alight intensity profile of the light source and D is a deviation rangingfrom zero to 20% of the maximum value of said curve, R_(max).

The inventors have realized that the non-uniform color distributionobtained in prior art lighting devices is caused by the non-uniformillumination of the wavelength converting layer by the light source.Light sources such as LEDs often have a Lambertian-like lightdistribution pattern, which means that the light intensity is higher inthe main forward emission direction, which is right above or in front ofthe light source, i.e. at a point opposite to a base at which the lightsource is mounted, than in the lateral directions. When using aconventional semi-circle-shaped wavelength converting layer as typicallyused in linear lighting devices, the less illuminated edges or near edgeregions of the wavelength converting layer have a slightly differentcolor compared to the more illuminated regions, which correspond to themid, or upper relative to a lower base at which the light source may bearranged, portion of the wavelength converting layer. The lessilluminated edges have a color closer to the color of the wavelengthconverting material while the more illuminated regions have a color moretowards the color of the LEDs. For example, if one or more blue LEDs anda yellow phosphor are used, the edges of the wavelength converting layerwill appear to be closer to yellow than the upper portion of the curvedwavelength converting layer, which will be appear to be closer to blue.

The illuminance E of the wavelength converting layer depends on thedistance Rbetween the light source and the wavelength converting layerand the luminous intensity profile I(φ) of the light source, whichprofile depends on the angle φ of a light path with respect to theoptical axis of the light source, according to the equation:

$\begin{matrix}{{E(\phi)} = \frac{I(\phi)}{R^{2}}} & ( {{Equation}\mspace{14mu} 2} )\end{matrix}$

If instead the illuminance E is held constant and the distance R isallowed to vary as a function of the angle, an equation is obtaineddefining a curve shape of the wavelength converting layer, which will bemore uniformly illuminated compared to a conventional wavelengthconverting layer not adapted to the luminous intensity distributionprofile of the light source. The distance may thus be defined as:

$\begin{matrix}{{R(\phi)} = ( \frac{I(\phi)}{E} )^{1/2}} & ( {{Equation}\mspace{14mu} 3} )\end{matrix}$

Equation 3 defines a curve on which the shape of the wavelengthconverting layer preferably may be based in order to obtain a moreuniform illuminance, and thereby a more uniform, or more out leveled,color gradient at the wavelength converting layer.

The present invention uses the concept of adapting the curve shape ofthe wavelength converting layer to the luminous intensity distributionof the light source such that the distance from the light source to thewavelength converting layer is shorter at angles φ where the luminousintensity is lower and longer at angles φ where the luminous intensityis higher. A curve shape of the wavelength converting layer as definedby Equation 1 is adapted to the luminous intensity distribution patternof the light source, whereby the wavelength converting layer is moreuniformly illuminated.

The present invention is thus advantageous in that the lighting deviceprovides a more uniform color distribution of emitted light across thewavelength converting layer and the risk for color gradients andartifacts is reduced. In addition, the far field luminous intensity ofthe lighting device is more uniform due to the more uniformlyilluminated wavelength converting layer.

A deviation, as defined by ±D in Equation 1, from the luminous intensityprofile based curve shape, k·I(φ)^(1/2), may be envisaged while stillproviding a more uniform illuminance of the wavelength converting layercompared to prior art. It will be appreciated that the deviation Dranging from zero to 20% of the maximum value of the curve R_(max) maybe constant or vary with the angle φ. Preferably, the deviation D mayrange from zero to 10%, even more preferably to 5%, of the maximum valueof the curve R_(max). Alternatively, the deviation D may range from zeroto 20% of R(φ).

It will be appreciated that the plane, which the wavelength convertinglayer intersects, is an imaginary, i.e. fictitious, plane extendingthrough the light source and being substantially parallel with theoptical axis of the light source. Further, in the present disclosure theoptical axis may be an axis extending through the light source and beingparallel with the main forward emission direction of the light sourcewhich typically, in particular for LEDs, is the direction at which theemitted light intensity is highest.

According to an embodiment, the wavelength converting layer mayintersect the plane with a curve given in a polar coordinate systemcentered at the light source by the equation:R(φ)=k·cos(φ)^(1/2) ±D  (Equation 4)

In case of a Lambertian-type light source the luminous intensity profilecan be defined as:I(φ)=I ₀·cos(φ)  (Equation 5)wherein I₀ is the luminous intensity of the light source at φ=0.Incorporating Equation 5 into Equation 2 shows that the maximumilluminance E_(max) of a conventional semi-circle-shaped wavelengthconverting layer is located opposite to or in front of the light source,close to φ=0, while the illuminance at the edges, close to φ=±90°, isnegligible and virtually zero. With the present embodiment, the curveshape of the wavelength converting layer is adapted to the luminousintensity distribution profile of a Lambertian-type light source.Incorporating Equation 5 into Equation 3 gives a definition of adistance according to Equation 6:

$\begin{matrix}{{R(\phi)} = {( \frac{I_{0}}{E} )^{1/2} \cdot ( {\cos(\phi)} )^{1/2}}} & ( {{Equation}\mspace{14mu} 6} )\end{matrix}$

Equation 6 defines a cosine based curve on which the shape of thewavelength converting layer preferably may be based for use incombination with Lambertian-type light sources in order to obtain a moreuniform illuminance, and thereby a more uniform, i.e. more out leveled,color gradient at the wavelength converting layer. The term (I₀/E)^(1/2)can be expressed as a constant k, whereby Equation 4 is provided fordefining a preferable curve shape of the wavelength converting layer.

According to an embodiment of the present invention, the wavelengthconverting layer may intersect the curve as defined by Equation 1 orEquation 4 at least from φ=−30° to φ=30°, preferably at least fromφ=−60° to φ=60°, and even more preferably at least from φ=75° to φ=75°.Hence, a considerable, and preferably a major, part of the wavelengthconverting layer follows the curve given by Equation 1 or Equation 4,and the wavelength converting layer is therefore more uniformlyilluminated as compared to prior art wavelength converting layers.

According to an embodiment of the present invention, the wavelengthconverting layer may intersect the curve at most from φ=80° to φ=80°.The present embodiment is advantageous in that the closest distance fromthe wavelength converting layer to the light source is increased,whereby a higher chemical stability of the wavelength convertingmaterial is obtained. Hence, the wavelength converting layer may notextend all the way to the light source, leaving a space between thelight source and the edges of wavelength converting layer. This isadvantageous since wavelength converting material located in the veryproximity of a light source tends to gradually deteriorate due to theheat generated by the light source and high energy light from the lightsource.

In an embodiment, the constant k may have a value comprised within theinterval 0.005 to 0.02 meter, which results in an appropriate shape ofthe wavelength converting layer, defined in meter, when using an LEDhaving a luminous intensity of around 5 lm to 200 lm at φ=0 as a lightsource. Preferably, the constant k may be higher when using a lightsource with higher luminous intensity and lower when using a lightsource with lower light intensity. For example, the value of theconstant k may be determined based on the desired illuminance E and theluminous intensity I₀ of the light source at φ=0 according to thefollowing equation:

$\begin{matrix}{k = ( \frac{I_{0}}{E} )^{1/2}} & ( {{Equation}\mspace{14mu} 7} )\end{matrix}$

As an example, when using T8 LEDs having a luminous intensity of around50 lm/LED, the constant k may be preferably around 0.0127 meter.

According to an embodiment of the present invention, the light sourcemay be configured to emit light with a Lambertian-like distribution,which implies a higher light intensity in the forward emission directionthan in the lateral directions. The light source may e.g. be aLambertian-type light source. The present embodiment is advantageous inthat the shape of the wavelength converting layer and the lightdistribution of the light source are better adapted to each other, i.e.match each other, whereby the illuminance of the wavelength convertinglayer becomes even more uniform. For example, the light source may be asolid state light source, such as an LED, which typically provides aLambertian-like light intensity distribution pattern.

According to an embodiment, the wavelength converting layer may comprisediffusing means whereby light from the light source is scattered into awider intensity distribution by the wavelength converting layer. Thediffusing means may be scattering particles, a scattering surfacestructure, e.g. a rough surface, and/or air voids in the wavelengthconverting layer. Alternatively, or as a complement, a separatediffusing layer may be arranged outside the wavelength converting layer,i.e. on the side of the wavelength converting layer not facing the lightsource. Such diffusing layer may e.g. be a holographically made diffusersurface or simply an optical layer comprising scattering particles or ascattering surface structure. In the present embodiments, the diffusingmeans may be anisotropic, which is advantageous for linear lightsources, wherein the diffusing means may be adapted to scatter light inthe length direction of the tube.

For shaping the light distribution, the lighting device may compriseoptical structures, such as prisms, preferably arranged outside thewavelength converting layer. Such optical structures may be adapted torefracting light in any desired directions.

According to an embodiment of the present invention, the lighting devicemay further comprise an envelope enclosing the light source and thewavelength converting layer, whereby the wavelength converting layer isbetter protected from damage. The envelope may have any desired shapeand may not necessarily follow the curve shape of the wavelengthconverting layer. Hence, the envelope may have e.g. a conventionalsemi-circular shape in case of a linear-type lighting device, wherebythe lighting device will have the appearance of a conventional lightingdevice. Optionally, the envelope may comprise diffusing means as thosedescribed in the preceding embodiment.

According to an embodiment of the present invention, a gap, such as anair gap, is defined between the wavelength converting layer and theenvelope, whereby the wavelength converting layer and the envelope maybe physically separated for disabling optical contact there between.Hence, the outer surface of the wavelength converting layer and theinner surface of the envelope may be physically separated for providingan air gap or a gap with any gas or vacuum. Alternatively, or as acomplement, the surface of the wavelength converting layer facing theenvelope may have an uneven surface structure, such as being rough,thereby reducing the optical contact between the wavelength convertinglayer and the envelope even if they about each other. In the presentdisclosure, the term “optical contact” means the physical contactbetween two optical bodies having similar refractive indices implyingjust a slight, i.e. negligible, or no refraction of light travelingacross the boundary between the two optical bodies. The optical contactbetween the wavelength converting layer and the envelope may preferablybe reduced, or even avoided, as it may influence the light distributionin terms of both intensity and color.

According to an embodiment, the lighting device may be a linear-typelighting device. The lighting device may hence have an elongated shapeand the light sources may be arranged in a row. Looking into a crosssection of such a linear-type lighting device, taken along a planeperpendicular to the longitudinal direction of the lighting device, thelight source is similar to a point-like light source, whereby theilluminance across the wavelength converting layer in the directionperpendicular to the longitudinal direction of the lighting device ismore uniform. Further, the wavelength converting layer may be elongatedand the plane, which the wavelength converting layer intersects, may beperpendicular to the longitudinal direction of the wavelength convertinglayer, thereby making illuminance of the wavelength converting layereven more uniform. It will be appreciated that the linear-type lightingdevice may have any desired shape as long as the light sources arearranged in a row, such as elongated and curved, or torus shaped.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present invention will now be described inmore detail, with reference to the appended drawings showing embodimentsof the invention.

FIG. 1 is a cross sectional view of a lighting device according to priorart.

FIG. 2 is a cross sectional view of a lighting device according to anembodiment of the present invention.

FIG. 3 shows a polar coordinate system in which a curve shape accordingto an embodiment of the present invention is represented.

FIG. 4 is a cross sectional view of a lighting device according toanother embodiment of the present invention.

FIG. 5 is a cross sectional view of a lighting device according to yetanother embodiment of the present invention.

All the figures are schematic, not necessarily to scale, and generallyonly show parts which are necessary in order to elucidate the invention,wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

With reference to FIG. 1, a lighting device according to prior art willbe described. FIG. 1 is a cross sectional view taken along a planeperpendicular to the longitudinal direction of a linear-type lightingdevice 1. The lighting device 1 comprises a blue LED 12, i.e. an LEDemitting blue light, a heat sink 13 with a cavity 14 for drivingelectronics (not shown) and a wavelength converting layer 11, which alsofunctions as an envelope enclosing the LED 12. The wavelength convertinglayer 11 comprises wavelength converting material, such as yellowphosphor, i.e. a phosphor emitting yellow light upon absorption ofphotons, preferably from the blue light of the LED 12, for providing acertain color of the light output from the lighting device 1. Thedistance from the LED 12 to the wavelength converting layer 11 isdenoted R and the angle with respect to the optical axis 10 of the LED12 is denoted φ. The cross section of the wavelength converting layer 11is semi-circular and the distance R is the same irrespective of theangle φ and, hence, constant across the wavelength converting layer 11.As LEDs typically have a Lambertian-type light intensity distribution,the wavelength converting layer 11 will be non-uniformly illuminatedwhen the LED 12 is turned on, whereby a color gradient across theenvelope will be visible. Typically, the portion of the wavelengthconverting layer opposite to or in front of the LED 12 will be more bluethan the near edge portions, which will be more yellow, due to thehigher light intensity of the LED 12 in the forward direction than inthe lateral directions.

With reference to FIG. 2, a lighting device according to an embodimentof the present invention will be described. FIG. 2 is a cross sectionalview taken along a plane perpendicular to the longitudinal direction ofa linear-type lighting device 2 such as a tube lamp. Light sources 22are arranged in a row or line in the lighting device 2, preferably witha pitch, i.e. a distance between the light sources 22, sufficientlysmall to reduce visible spots at the surface of the envelope of thelighting device 2. As the cross sectional view in FIG. 2 is takenperpendicular to the longitudinal direction of the linear-type lightingdevice 2, only one light source 22 is visible in the figure.

The lighting device 2 further comprises a heat sink 23 defining a cavity24 in which the electronics (not shown) for driving the light sources 22are arranged, a wavelength converting layer 21 and an envelope 25enclosing the wavelength converting layer 21 and the light sources 22.The wavelength converting layer 21 comprises wavelength convertingmaterial, or luminescent material, such as phosphor pigments (e.g.YAG:Ce) and/or luminescent dye for converting the wavelength of thelight from the light sources 22 into a desired color.

The shape of the wavelength converting layer 21 is advantageouslyadapted to the luminous intensity distribution pattern of the lightsource so as to obtain a more uniform illuminance of the wavelengthconverting layer 21 than that obtained in the prior art device describedwith reference to FIG. 1. In the present embodiment, the wavelengthconverting layer 21 intersects a fictitious plane extending through thelight source 22 and being parallel with the optical axis 20 of the lightsource 22, at a curve given, in a polar coordinate system centered atthe light source 22, by the equation:R(φ)=k·cos(φ)^(1/2) ±D  (Equation 4)wherein k is a constant, φ is an angle with respect to the optical axis20 and D is a deviation ranging from zero to 20% of the maximum value ofthe curve, R_(max). The plane which the wavelength converting layer 21intersects is, in the present example as shown in FIG. 2, perpendicularto the longitudinal direction of the linear lighting device 2 and thusparallel with the plane at which the cross section is taken in thefigure. The constant k may be set to a value adapted for obtaining anappropriate size of the wavelength converting layer 22 and/or anappropriate distance from the light source 22 to the wavelengthconverting layer 21. For example, the value of the constant k may bebased on the desired illuminance E at the wavelength converting layer 22and the far-field luminous intensity I₀ of the light source 22 at φ=0according to the equation:

$\begin{matrix}{k = ( \frac{I_{0}}{E} )^{1/2}} & ( {{Equation}\mspace{14mu} 7} )\end{matrix}$

FIG. 3 shows the curve 32 as defined by Equation 1 represented in apolar coordinate system. In the present non-limiting illustrativeexample, the constant is set to k=1 and the deviation is set to D=0. Ascan be seen in both FIG. 2 and FIG. 3, the maximum distance, representedby the maximum value of the curve R_(max) from the light source 22,positioned at the pole of the polar coordinate system, to the wavelengthconverting layer 21 is in front of or above the light source 22 at φ=0where also the light intensity from the light source 22 is the highest,while the distance from the light source 22 to the wavelength convertinglayer 21 is at least near zero at φ=90° and φ=270° (also referred to asφ=−90° in the present disclosure), at which angles also the lightintensity from the light source 22 is the lowest.

For comparison, a curve 31 representing the shape of the prior artwavelength converting layer, as described with reference to FIG. 1, isalso represented in the polar coordinate system. As can be seen in bothFIG. 3 and FIG. 1, the distance between the light source 12 and thewavelength converting layer 11 represented by curve 31 is constant fromφ=90° to φ=270°. The equal distance at low and high angles implies thatthe illuminance of the wavelength converting layer 11 will be relativelyhigh at low angles, i.e. close to φ=0, and relatively low at highangles, i.e. close to φ=90° and φ=270°.

With reference to FIG. 4, an embodiment of the present invention will bedescribed. FIG. 4 shows a lighting device 4 similar to the lightingdevice 2 described with reference to FIG. 2, with the difference thatthe heat sink 43 is arranged such that it shadows less light from thelight source 42, wherein the light source 42 is slightly elevated withrespect to the heat sink 43. With the present embodiment, the lateralextension or width of heat sink 43 is reduced such that more light isemitted backwardly relative to the forward emission direction parallelwith the optical axis 40 of the light source 42. Hence, a moreomni-directional light distribution is obtained. Further, a base atwhich the light sources 42 are arranged is covered by a reflector 46,which may be diffuse or specular, for increasing the light output fromthe lighting device 4. The wavelength converting layer 41 may beconfigured as in the embodiment described with reference to FIG. 2. Theenvelope 45 is arranged to cover the wavelength converting layer 41 andthe light source 42.

With reference to FIG. 5, another embodiment of the present inventionwill be described. FIG. 5 shows a lighting device 5 similar to thelighting device 2 described with reference to FIG. 2, with thedifference that the wavelength converting layer 51 intersects the curvedefined by Equation 1 at a narrower angle interval. Preferably, thewavelength converting layer 51 may intersect the curve at least fromφ=−30° (also referred to as φ=330°) to φ=30°, preferably from φ=−60°(also referred to as φ=300°) to φ=60°, and even more preferably at leastfrom φ=−75° (also referred to as φ=285° to φ=75°, with respect to theoptical axis 50 of the light source 52. However, in the presentembodiment, the wavelength converting layer 51 may intersect the curveat most from φ=−80° (also referred to as φ=280°) to φ=80°. The morelimited coincidence with the curve provides a space between the lightsource 52 and the edges 57 of the wavelength converting layer 51, i.e.the edges or end points at which the curved shape as defined inaccordance with Equation 1 terminates, and the closest distance from thewavelength converting layer 51 to the light source 52 is increasedcompared to the embodiment described with reference to e.g. FIG. 2. Asheat generated by the light source 52 may in time gradually deterioratethe stability of the phosphor composition in the wavelength convertinglayer 51, it is advantageous to separate the edges 57 of the wavelengthconverting layer 51 from the light source 52 and the heat sink 53.Between the edges 57 of the wavelength converting layer 51 and the baseplate at which the light source 52 is arranged, reflectors 56, e.g.diffuse or specular, or translucent diffusers (not shown) may bearranged for supporting the wavelength converting layer 51 forincreasing the light output from the lighting device 5.

In the following, further embodiments of the invention, which may becombined with any one of the previously described embodiments, will bedescribed.

Preferably, the ratio between the pitch p and the maximum distance fromthe light source to the wavelength converting layer R_(max) isR_(max)/p≧1 for providing a more uniform color distribution orconversion along the linear lighting device. Further, the light sourcesmay preferably be equally spaced in the row configuration.

The wavelength converting layer may comprise diffusing means, such asscattering particles, e.g. TiO₂ or Al₂O₃, air voids and/or a scatteringsurface structure. The diffusing means may be arranged within thewavelength converting layer or as a separate layer coated on thewavelength converting layer. Diffusing means may alternatively, or as acomplement be arranged at the envelope for further smoothening any colorirregularities or artifacts present at the wavelength converting layerand thereby in the light intensity distribution. Further, the wavelengthconverting layer and/or the envelope may comprise optical structures,such as prisms, lenticulars or holographically made structures, forimproving the color uniformity and/or spread the light in desireddirections to tune the far field intensity distribution of the lightingdevice. For reducing the quality of the optical contact between thewavelength converting layer and the envelope, the outer surface of thewavelength converting layer and/or the inner surface of the envelope 25may be rough, at least in the region where the two optical parts abouteach other. Alternatively, an air gap may be defined between thewavelength converting layer and the envelope for avoiding opticalcontact. Furthermore, the wavelength converting layer and/or theenvelope may be extruded optical covers, i.e. manufactured by extrudingsoft material through an opening having the desired profile, with auniform thickness or a variation in thickness dependent on the angle φ.

The person skilled in the art realizes that the present invention by nomeans is limited to the preferred embodiments described above. On thecontrary, many modifications and variations are possible within thescope of the appended claims. For example, the examples of curve shapeand size of the wavelength converting layer, as well as otherconstituent parts of the lighting device described with reference toFIG. 2 is also applicable in any of the other described embodiments.

The invention claimed is:
 1. A lighting device comprising: a wavelengthconverting layer having a curved shape, and a light source arranged toemit light towards the wavelength converting layer, wherein thewavelength converting layer intersects a plane extending through thelight source and being parallel with the optical axis of the lightsource, at a curve given, in a polar coordinate system centered at thelight source, by the equation:R(φ)=k·cos(φ)^(1/2) ±D, wherein k is a constant, φ is an angle withrespect to said optical axis, I(φ) defines a luminous intensity profileof the light source and D is a deviation ranging from zero to 20% of themaximum value of said curve, R_(max).
 2. The lighting device as definedin claim 1, wherein the wavelength converting layer intersects saidcurve at least from φ=−30° to φ=30°, preferably at least from φ=−60° toφ=60°, and even more preferably at least from φ=−75° to φ=75°.
 3. Thelighting device as defined in claim 1, wherein the wavelength convertinglayer intersects said curve at most from φ=−80° to φ=80°.
 4. Thelighting device as defined in claim 3, wherein the constant (k) has avalue comprised within the interval 0.005 to 0.02 meter.
 5. The lightingdevice as defined in claim 4, wherein the light source is configured toemit light with a Lambertian-like distribution.
 6. The lighting deviceas defined in claim 5, wherein the wavelength converting layer comprisesa diffusing means.
 7. The lighting device as defined in claim 5, furthercomprising an envelope enclosing the light source and the wavelengthconverting layer.
 8. The lighting device as defined in claim 7, whereina gap is defined between the wavelength converting layer and theenvelope.
 9. The lighting device as defined in claim 7, wherein thesurface of the wavelength converting layer facing the envelope has anuneven surface structure.
 10. The lighting device as defined in claim 9,wherein the lighting device is a linear-type lighting device.
 11. Thelighting device as defined in claim 10, wherein the wavelengthconverting layer is elongated and said plane is perpendicular to thelongitudinal direction of the wavelength converting layer.